One important control for improving the efficiency of a wind turbine generator system is yaw control in which the direction of the wind turbine rotor is controlled in accordance with the wind direction. The wind turbine generator system, which provides highest efficiency when the wind turbine rotor faces the front with respect to the wind, requires direction control of the wind turbine rotor by performing a yaw rotation of the nacelle which supports the wind turbine rotor in accordance with the wind direction. Various approaches have been made for yaw rotation mechanisms and yaw control techniques; for example, Japanese Laid Open Patent Application No. P2004-285858A discloses a technique in which the wind direction and wind power are detected with the use of a laser anemovane and yaw control is performed based on the detected wind direction and wind speed. Additionally, Japanese Laid Open Patent Applications Nos. P2005-113899A and P2001-289149A disclose a configuration of a drive mechanism for the yaw rotation of the nacelle.
One important issue of the yaw control of the wind turbine generator system is to minimize the number of times of yaw rotations of the nacelle. Due to the large weight of the nacelle, a large number of times of yaw rotations of the nacelle cause increased mechanical loads of the rotation mechanism which rotates the nacelle and the braking mechanism which stops the rotations of the nacelle, increasing mechanical wear of these mechanisms. In order to reduce the wear of the rotation mechanism and braking mechanism, it is desirable that the number of times of yaw rotations be reduced.
The most general control logic of the yaw control used to satisfy such need is a control logic in which, when a state in which the absolute value of the wind direction deviation, that is, the deviation between the wind turbine direction (i.e. the direction of a wind turbine rotor) and the actual wind direction, is greater than a predetermined threshold value continues for a predetermined duration time (e.g. 20 seconds), a yaw rotation of the nacelle is performed such that the wind direction deviation is zero (i.e. such that the wind turbine direction agrees with the most recent wind direction), as shown in FIG. 1. Such a control logic, in which a yaw rotation is not performed unless the absolute value of the wind direction deviation exceeds a threshold value, reduces the number of times of yaw rotations by setting an appropriate threshold value.
One problem of such control logic is that the value of the wind direction deviation is not reduced averagely under a condition where the wind direction gradually changes over a long time (over several hours under some wind conditions), as shown in FIG. 2. Depending on locations of mountains, valleys and seas, there is a case where a wind condition at a certain point shows random changes in the wind direction with high degree of randomness during the daytime but does not show random changes in the wind direction at nighttime. In other words, the wind condition at nighttime often exhibits wind direction changes over a long time. The use of the above-mentioned control logic averagely reduces the value of the wind direction deviation close to zero under a condition where the wind direction randomly changes with high degree of randomness. However, when the wind direction gradually changes over a long time (over several hours under some wind conditions) (indicated by “A” at the top of FIG. 2) as shown in FIG. 2, the wind direction deviation becomes zero only for a moment (C at the bottom of FIG. 2) even if yaw rotations are repeated (indicated by “B” at the middle of FIG. 2) in the case of the above-mentioned control logic. Therefore, the average value of wind direction deviations is not reduced. This is not preferable in terms of improvement of the efficiency of a wind turbine generator system.